The present invention relates to a digital magnetic-recording apparatus and, more particularly, to a digital data reproducing circuit for use in a digital magnetic-recording apparatus.
A recording apparatus of the type using a magnetic disk, magnetic tape or similar magnetic recording medium, reproduces stored digital data in the form of an analog waveform representative of transitions of magnetic polarity on the recording medium. In the case of data recorded by an NRZI (non-return-to-zero) modulation system, for example, a data "1" is representative of a positive or a negative peak of the reproduced waveform. A digital data reproducing circuit, therefore, differentiates the reproduced waveform and determines the points where the differentiated waveform crosses a zero level representative of zero AC volts, to detect the peaks.
When a recording medium has a magnetic defect such as a change in the density of the magnetic powder or when electrical noise is introduced in a playback system from outside the system, false data generally referred to as a "drop-in" and the omission of data generally referred to as a "drop-out" can occur. To eliminate such occurrences, it has been customary with conventional data reproducing circuits to detect the absolute amplitudes of a reproduced analog waveform and to convert, among the above-mentioned peaks, only the peaks having amplitudes higher than a predetermined threshold level into digital signals, determining that they are data "1".
However, the conventional data reproducing circuits have a critical drawback that when external noise whose amplitude exceeds the threshold level is introduced in the reproduced analog signal, the circuits translate even the noise into data "1".